1. Field of the Invention
This invention relates to apparatus for automatically controlling the temperature of welding bands used to weld plastic films or the like. Such weld bands are commonly found on plastic bag making machines. The inventive apparatus basically comprises a dividing circuit for determining the temperature of the welding band in dependence on the output voltage and current of the circuit used to heat the welding band and a controller for controlling the output power of the heating circuit in dependence of the deviation of the actual temperature of the welding band from a predetermined desired value.
2. Description of the Prior Art
In the operation of known apparatus for controlling the temperature of welding bands, such as is known from U.K. Patent Specification No. 1 520 556 (incorporated by reference herein), a recognized phenomenon is utilized. Accordingly, the resistance of the sealing band increases with temperature so that the temperature of the welding band can be determined in dependence on its resistance (.theta.W =f(Us/Is)), wherein .theta.W is the temperature of the hot welding band and Us and Is are the output voltage and current, respectively, of the heating circuit. In the known apparatus the temperature of the welding band is determined with reference to its actual resistance. A dividing circuit causes a controller to control the current flowing in the heating circuit so that the current supplied to the welding band will maintain the welding band at the desired, constant temperature.
The known apparatus will operate satisfactorily if the welding bands employed have the same geometric configuration and the same composition. In that case the parameters and properties of the welding bands can be taken into account in the measurement of the actual values and the determination of the desired value.
But in fact, welding bands differ in geometrical dimensions and in composition so that expensive adjusting operations are required in the known apparatus when a welding band has been replaced if the actual temperature of the new welding band is to be controlled to match the predetermined desired temperature with adequate accuracy.